Optical fiber coupled devices and method of assembling same

ABSTRACT

An optical device, e.g. a signal generator adapted to provide an optical signal to a fibre tail or to a socket adapted to receive a fibre tail, is assembled by two thin films of adhesive. One film is normal to the beam path and, preferably 50-200μ thick. The other film is parallel to the beam path, e.g. surrounding the beam path, and preferably 30-90μ thick. Conveniently the device includes an intermediary component and end of said film adheres one of said components to said intermediary component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical devices and more particularly to anassembly for optical devices comprising two components which arerequired to be positioned with respect to each other so as to providefor the efficient transmission of optical signals from one to the other.

2. Related Art

The invention has an important application in the manufacture of opticalcommunications apparatus where, for example, accurate and stablerelative positioning is required between two components. For the presentpurposes it is convenient to identify one of the components as a sourcecomponent and the other as a receptor component. It is a commonrequirement in optical communications to juxtapose these so that thereceptor component is accurately located so as to acquire opticalsignals provided by the source component. For example, one of thecomponents may comprise a passive optical device in the form of opticalfibre waveguide, and the other component an active optical device suchas a laser, photo diode. Accurate and stable relative positioningbetween the source component and the receptor component is required toensure optimal and stable optical alignment of the optical devices.Similarly, such accurate and stable relative positioning may be requiredbetween two components comprising active devices or two componentscomprising passive devices.

It is now convenient to identify the various forms of optical deviceswhich source components and receptor components commonly comprise.

The source component may comprise a signal generator, e.g. asemiconductor device which converts electric power into an opticalsignal. Examples of such semiconductor devices include light emittingdiodes (LED), edge emitting LEDs (ELED), and a wide variety ofsemiconductor lasers. Many of these devices comprise waveguidingstructures. The signal generator often provides a divergent beam whichis inconvenient for forming an optical connection. In such circumstancesit is usual for the source component to include lenses which eithercollimate the beam or to cause it to converge to a focus. The sourcecomponent may also comprise a passive device, e.g. a passive opticalwaveguiding structure such as an optical fibre.

The receptor component may be an optical detector, e.g. a semiconductordevice which converts an optical signal into an electrical signal.Examples of such devices include photo diodes and PIN diodes. Thereceptor component may also comprise a waveguiding structure, e.g. apassive waveguiding structure such as an optical fibre, which is adaptedto receive an optical signal from the source component for onwardtransmission to a receiving station.

It should be noted that passive waveguiding structures can be utilisedeither in the receptor component (e.g. an optical fibre for transmittingthe output of an optical transmitter) or in the source component (e.g.an optical fibre terminating at an optical receiver).

Optical isolators are a well known example of passive optical componentsof the type in which a waveguide structure need not be employed.

It has been mentioned above that one of the components may comprise awaveguiding structure. In some cases the end of an external waveguidingdevice may be permanently comprised in the component, e.g. a fibre tailmay be permanently located for the transmission of the optical signal.However, optical technology includes a wide range of connectors, e.g.connectors in which a plug, connected to a first fibre, is inserted intoa socket, connected to a second fibre. In other embodiments of theinvention the waveguiding structure takes the form of a plug or a socketwhich is adapted to connect to an external waveguiding device. When theconnection is completed the external waveguiding device is operativelyconnected to the internal device. The location of the plug or the sockettherefore (indirectly) locates the external waveguiding device and thisis an important embodiment for optical devices in accordance with theinvention.

It is emphasised that, in all the cases mentioned above, a satisfactoryperformance of assembly depends upon the accurate relative location ofthe two components whatever form these components may take. It will alsobe appreciated that, whatever the optical function, the components oftencomprise similar, or identical cases, and, therefore, the mechanicalproblem of the assembly of the devices is independent of the opticalfunction.

Typically, the optical tolerances are less than ±3 μm and preferablyless than ±1 μm. In the case where one of the components includes asocket or plug for making connections to external waveguides, e.g.fibre, it will be apparent that there is a variability in the overallperformance of the assembly because the plug does not always go intoexactly the same position in its socket. The plug and socket aredesigned .so that this error is small but it is necessary that thecombined error be kept within the tolerances mentioned above. This meansthat the relative location of the two components is always subject totight tolerances which must be maintained within the lifetime of thedevice.

The basic method of assembling the devices is well established. Thesource component is activated and the relative positions of the sourcecomponent and the receptor component are experimentally adjusted untilthe receptor component acquires maximum output. With the two componentsin this optimum configuration, they are secured by the application ofadhesive. Many jigs are available which allow this process to be carriedout and the jigs are capable of holding the two components in accuratejuxtaposition so long as both remain within the jig. It has, however,been observed that the performance of the assembly deteriorates morerapidly than the lifetimes of its individual components would imply. Itis an object of this invention to reduce the rate of this deterioration.

BRIEF SUMMARY OF THE INVENTION

According to this invention the components of an assembly, e.g. a sourcecomponent and a receptor component, are held together by a thin film ofadhesive or by thin films of adhesive. Preferably the assembly includestwo forms of adhesive (both of which are thin) e.g. a first film whichis normal to the beam path and a second film which is parallel to thebeam path. Conveniently the second film has a tubular configurationwhich surrounds, and is parallel to, the beam path. The first film ofadhesive is preferably less than 250 μm thick, e.g. 50-150 μm thick. Inthe case of the second film of adhesive the preferred thickness is lessthan 150 μm, especially less than 100 μm, e.g. 30-90 μm. It has beenfound that device assemblies using thin films of adhesive as describedabove retain their performance longer than conventional deviceassemblies which have thick films of adhesive. It is believed that thedeterioration in the performance of conventional device assemblies isdue to dimensional instability in the adhesive films. Because deviceassemblies according to the invention have thin films the dimensionalinstability of the thin films has a smaller effect than in theconventional device assemblies.

Device assemblies according to the invention therefore have twocomponents which are accurately retained in position by thin films ofadhesive. In order to achieve the necessary accurate location, it isconvenient to utilise an intermediary component wherein each componentis separately secured to the intermediary component by means of its ownthin film of adhesive. The intermediary component preferably has acylindrical configuration having a cylindrical wall which is parallel tothe beam path and an end face which is normal to the beam path. The endface has an aperture to permit the passage of the optical signal. In thefinished assembly the end face is secured to one of the components bymeans of the first film of adhesive and the cylindrical wall is securedto the other component by means of the second film of adhesive. It willbe appreciated that components are usually assembled inside a case whichalso has a cylindrical wall and an end face with an aperture for thepassage of the optical signal. The components are, therefore,geometrically compatible with the intermediary component. It is usuallyconvenient for one of the components to be located at least partiallyinside the intermediary component in which case the thickness of thesecond film of adhesive is determined by the thickness of the annulus soformed.

The components and the intermediary component according to the inventioncan be assembled using techniques very similar to those described above.That is one component is used to support the intermediary component andthen the components are adjusted into the correct relationship using ajig as described above. During this adjustment the intermediarycomponent is not attached to the jig whereby it is free to adopt its ownlocation. When the optical signal indicates that the components aresuitably aligned a small amount of adhesive is placed on the gap normalto the beam path and surface tension draws the adhesive into the gap andthe surface tension also draws the intermediary component into a closerelationship so that the first film of adhesive, having a thicknesswhich lies within the range specified, is formed. After the first dropof adhesive has been applied a second drop of adhesive is applied intothe other gap which is between the intermediary component and the othercomponent. This gap is parallel to the beam path. Surface tension drawsthe adhesive into this gap and it tends to center the intermediarycomponent (which is free to move) so that a uniform thin film ofadhesive, i.e. the second film of adhesive, is achieved. The second dropshould be applied before the first adhesive has hardened whereby thefirst adhesive does not restrict the centering of the second adhesive.Both sets of adhesive are allowed to set while the source component andthe receptor component are held in the correct relationship by the jigand, when both adhesives have set, the correct relationship is madepermanent with thin films of adhesive as described above. The thicknessof the second film of adhesive is determined by the thickness of theannular gap and the components are sized so that this thickness lieswithin the specified thickness range for the second film of adhesive.

It will be appreciated that the configuration and techniques describedabove do not restrict the freedom of relative movement for the initialadjustment of the source component and the receptor component eventhough the thickness of the adhesive films present in the final deviceare subject to very stringent thickness specifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a device in accordance withthe invention,

FIG. 2 is a diagrammatic representation showing the device of FIG. 1 inthe course of assembly,

FIG. 3 is a longitudinal cross section illustrating a device in whichthe receptor is a socket, and

FIG. 4 is a device similar to FIG. 3 but the receptor contains a fibre.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates an assembly 1 in which two components 10 and 11 areadhered to an intermediary component 12. All three components havecylindrical surfaces and, in particular, cylindrical component 11 islocated inside the cylindrical part of the intermediary component 12 sothat there is a small annular gap which is preferably about 75μ thick.Component 10 has a surface 15 which is normal to the cylindrical axisand this surface 15 is situated close to an end surface 16 which isnormal to the cylindrical axis and is part of the intermediary component12. The gap between surfaces 15 and 16 contains a first thin film 13 ofan adhesive which secures component 10 to component 12. The thickness ofthe first film 13 is preferably about 200μ thick.

The annular space between the intermediary component 12 and thecomponent 11 also contains a second film of adhesive 14 and this filmsecures component 11 to intermediary component 12 whereby therelationship between components 10 and 11 is fixed. Because the externaldiameter of component 11 and the internal diameter of intermediarycomponent 12 are both made to accurate specifications the size of theannulus is fixed and hence the thickness of adhesive film 14 is alsocontrolled to the preferred thickness of about 75μ as stated above.

It will be appreciated that the structure illustrated in FIG. 1maintains components 10 and 11 in an accurate configuration over longperiods of time because the only elements liable to change theirdimensions are the two adhesive films 13 and 14 and, because these arethin, and only change in the thickness of the films affect the relativepositioning of the components. Therefore, even over long periods thechanges that occur are likely to be only very small and acceptable. Thisis true even though the tolerances are less than ±3μ and it is stillpossible to achieve tolerances as low as ±1μ.

It will be noted that FIG. 1 is asymmetrical in that component 11 iscontained within the intermediary component 12 whereas component 10 isnot. One of components 10 and 11 represents a source component and theother represents the receptor component. In spite of the asymmetry justmentioned it is not important which way round the assembly isconfigured. It will be noted that the beam of radiation travels alongthe center of the device and that all the components have windows in thecenter to permit this transmission. In general, the beam path willapproximately coincide with the geometrical axes of the variouscylinders.

FIG. 2 illustrates the important features of the technique forassembling the assembly illustrated in FIG. 1. As a preliminary toassembling, component 11 is introduced into the cylindrical portion ofintermediary component 12. Thereafter component 11 is offered up tocomponent 10 but the intermediary component 12 is not gripped so that itis completely free to move except as limited by the presence ofcomponent 11 in its cavity. Thus the two components 10 and 11 can bemanipulated into their intended relationship and the fact that theintermediary component 12 is not separately constrained means that itdoes not restrain or hinder the achievement of the correct relationshipbetween components 10 and 11. When the components 10 and 11 have beenplaced into their accurate alignment a small drop of adhesive isintroduced into the gap 17. The adhesive is a liquid at this stage andthe gap 17 is sufficiently narrow for the drop to be drawn into the gapby wicking action and, once the liquid adhesive is in the gap, thesurface tension of the liquid draws the intermediary component 12towards the component 10 whereby the thickness of the gap is establishedby the surface tension. This ensures that the thickness of the adhesivefilm is within the limits specified above.

After the first drop of liquid adhesive has been applied a second dropis introduced into the annulus 18 between component 11 and theintermediary component 12. The thickness of this annulus is controlledby the size of the two components and it is sufficiently small for thedrop to be drawn into the annulus by wicking action and thereaftersurface tension tends to center component 11 within the annulus. Thevarious components are preferably warmed, e.g. by using a heated jig, sothat the adhesive hardens and sets within a period of about 2 minutes.However, during the early part of this period the two films of adhesiveare sufficiently mobile to allow intermediary component 12 to move inaccordance with any changes in the size of the adhesive. During thistime components 10 and 11 are clamped in the jig so that they are heldin their desired relationship and the movement of the intermediarycomponent 12 does not affect the important adjustment. Once the twofilms of adhesive have dried the components 10 and 11 are fixedpermanently in their optimum relationship and, because the films arethin, this accurate relationship is retained throughout the life of thecomponent.

FIG. 3 shows the configuration of FIG. 1 applied to an optical assembly3 in which a source component 31 corresponds to the first working device11 and a socket 30 for accepting an optical fibre connector (not shown)corresponds to the second working device 10. The socket 30 is adapted toreceive a plug attached to a fibre to form a conventional fibreconnector and this socket 30 constitutes the receptor component.Moreover, the whole assembly is mounted within an exterior housing 39.

It can be seen that the optical assembly shown in FIG. 3 has the samestructure as illustrated in FIG. 1 for achieving long term dimensionalstability.

Assembling the assembly proceeds in the same Way as described withreference to FIG. 2. Assembling is carried out on a jig which includes aconvenient length of fibre terminated with a suitable plug at one endand connected to an intensity monitor (not shown) at the other. Havingplaced the source component 31 in the intermediary component 12, thelight source 38, such as an ELED, for example, in the source component31 is activated whereby a beam of light falls on the end of the fibreplugged into the socket 30. Conveniently the socket is clamped immovablyand all the adjustment is achieved by moving the source component 30until the maximum intensity is recorded. At this point the drops ofadhesive are applied as described with reference to FIG. 2 and theconfiguration is made permanent.

FIG. 4 illustrates a device in which the source component 40 isjuxtaposed to the end of a fibre 41 which is permanently constituted inthe receptor device. In this configuration the source component 40corresponds to component 10 of FIG. 1 and the receptor componentcorresponds to component 11. The thick-walled sleeve 42 takes the placeof the cup-shaped intermediary component 12 of FIGS. 1 and 2. Theassembly is put together in substantially the same manner as that ofFIG. 3, with adhesive being introduced into gaps 43 and 44 whilecomponents 40 and 41 are clamped in it suitable jig. As before, theintermediary component is free to move subject to the same restraints asbefore, and is located in the proper position by the surface tension ofthe adhesive. It should be noted that in the case of FIG. 4, the housing47, although similar in appearance to the intermediary component ofFIGS. 1 to 3, forms instead on of the components 10 or 11, the activedevice 48 having been previously secured firmly within the cup-shapedcasing 46 in such manner that only minimal, and ideally no movement canoccur between the housing 47 and the casing 46.

I claim:
 1. An assembly for optical devices comprising components whichare secured in accurate alignment for the transmission of opticalsignals by thin films of adhesive having thickness of less than 250μ. 2.An assembly according to claim 1 in which there is a first thin film ofadhesive normal to the beam path and a second thin film of adhesiveparallel to the beam path.
 3. An optical device according to claim 2,wherein the first film of adhesive is 50-200μ thick.
 4. An assemblyaccording to claim 2, wherein the second film of adhesive is less than150μ thick.
 5. An assembly according to claim 4, wherein the second filmof adhesive is 30-90μ thick.
 6. An assembly according to claim 5,wherein the second adhesive film takes the form of a tube surroundingthe beam path.
 7. An assembly according to claim 2, wherein said devicecomprises two components namely, a source component adapted to provide abeam of light to a receptor component adapted to receive said beam oflight, wherein each of said source component and said receptor componentis adhered to an intermediary component whereby they are held ineffective adjustment for the transmission of said beam wherein saidintermediary component has a first surface normal to said beam path anda second surface parallel to said beam path wherein said first surfaceis adhered to one of said components by said first film of adhesive andthe other of said components is adhered to the intermediary component bysaid second film of adhesive.
 8. A method of assembling an opticaldevice having components which are secured in accurate alignment for thetransmission of optical signals by thin films of adhesive, a first thinfilm of adhesive being normal to an optical beam path and a second thinfilm of adhesive being parallel to the optical beam path wherein saiddevice comprises two components namely, a source component adapted toprovide a beam of light to a receptor component adapted to receive saidbeam of light, wherein each of said source component and said receptorcomponent is adhered to an intermediary component whereby they are heldin effective adjustment for the transmission of said beam wherein saidintermediary component has a first surface normal to said beam path anda second surface parallel to said beam path wherein said first surfaceis adhered to one of said components by said first film of adhesive andthe other of said components is adhered to the intermediary component bysaid second film of adhesive, said method comprising the steps of:(a)supporting the intermediary component on one of the components, (b)adjusting the relative position of the components for the satisfactorytransference of optical signals, wherein the movement of theintermediary component is not constrained except by said support, (c)introducing portions of adhesive into each of the gaps between saidintermediary component and each of said components wherein saidintroduction is substantially achieved by wicking action, and (d)holding said components into the position achieved in step (b).
 9. Amethod as claimed in claim 8 wherein the assembly takes place in aheatable jig.
 10. An assembly of optically coupled components secured inaccurate optical alignment by thin film adhesive having a thickness ofless than 250μ.
 11. An assembly as in claim 10 wherein said componentsare secured by first and second mutually orthogonal thin adhesive filmswith respect to an intermediate structure.
 12. An assembly as in claim11 wherein one of said orthogonal thin adhesive films is thinner thanthe other.
 13. An assembly as in claim 11 wherein said orthogonal thinadhesive films are respectively perpendicular and parallel to an opticalbeam coupling path between said components.
 14. An assembly as in claim13 wherein the thin adhesive film parallel to said coupling path isthinner than the thin adhesive film perpendicular to said coupling path.15. A method for assembling first and second optical components inaccurate alignment for transmission of optical signals therebetween overa beam coupling path, said method comprising the steps of:(a) assemblingsaid optical components in juxtaposition with an intermediate member,there being an adjustable gap between the intermediate member and eachof said optical components; (b) adjusting the relative position of thecomponents while so assembled for the satisfactory transference ofoptical signals; (c) introducing a quantity of adhesive into said gapswherein said introduction is substantially achieved by wicking action;and (d) holding the components in the position achieved by step (b)while the adhesive cures.
 16. A method as in claim 15 wherein theassembly is performed in a heated jig.